In prior art circuits of this type, for example as disclosed in the German Patent Application No. 29 47 881 delays of pulse edges or pulses are produced by means of, for example, monostable multi-vibrator circuits and the amplitude of the composite pulse to be tested is measured at two different moments. Thus it is determined whether the amplitude value at the two moments is higher or not higher compared with a limit value; from this it is determined whether a long pulse is present. As measurements are performed at only two different moments it is not impossible that two separate, shorter pulses occurring at these two moments are regarded as a long pulse.
The invention has for its object to enable an accurate measurement by means of the circuit arrangement of the real duration of the pulses to exclude the possibility of errors, so that a reliable field synchronizing signal is obtained. In addition, the circuit arrangement must be suitable for implementation as an integrated semiconductor circuit.